High-fidelity synthesis of microhole templates with low-surface-energy-enabled self-releasing photolithography

Material patterning through templates has provided an efficient way to meet the critical requirement for surface function in various fields. Here, we develop a self-releasing photolithographic process to make large-area freestanding templates with precise patterns. The low surface energy of substrates by hydrophobic treatment with proper silane modification ensures the template self-releasing. This method eliminates the need of mechanical separation or any sacrificial layers. Major steps including UV exposure and baking are optimized to realize high-quality structures and the final release of templates. The negative photoresists of SU-8 and polyimide are chosen to confirm the feasibility of this process. Wafer-scale freestanding templates with uniform microhole arrays are obtained with high structural fidelity, smooth surfaces and excellent flexibility. The hole size ranges from several to several tens of micrometers with an extremely low variation (<1%). These advantages could promote the application of precisely structured templates for surface patterning in material and surface science.


Introduction
As a versatile surface engineering technique, patterning with structured templates is of fundamental importance in the elds of material science, biotechnology and nanotechnology.Specically, freestanding templates based on thin membranes with well-dened micro/nanoholes are highly demanded as shadow masks for material synthesis in the biological assay of cells, DNA and proteins, 1-3 chemical assisted emulsication, 4,5 and nanomaterial patterning on arbitrary substrates otherwise incompatible with conventional techniques. 6,7iverse materials have been patterned onto target surfaces by synthesis and deposition through templates.For instance, a dry li-off procedure with a elastomeric template was demonstrated to pattern metals, organometallic molecules, biological macromolecules, and sol-gels. 8With a solithographic template/stencil sealed on the substrate, topologically designable microstructures were realized with chemical vapour deposition (CVD) and reactive coating deposition. 9,10etallic nanohole arrays were produced by a template-transfer procedure with nanostructured Si templates for plasmonic sensing. 11,12SU8 templates with micro-posts were utilized for nanoparticle tracking under optical microscope at biomolecular and cellular levels. 13Beyond the use in fabrication, structured templates are also incorporated into microuidic system to act as key components such as separators, 14,15 multiplexors, 16 and controllable interconnects for different channels/levels of microuidic arrays. 17dvance of fabrication technology has continually driven the evolution of template modality.Typical membrane templates made by phase separation usually suffer from irregular porous structures in random distribution. 18In contrast, the tracketched 19,20 and anodic aluminium oxide (AAO) 21,22 membranes feature the narrower hole-size distribution and lower tortuosity.However, the use of nuclear ssion or corrosive chemicals induces laborious procedures and undesirable contaminations.35 However, one particular difficulty lies in the separation of templates from the master structures due to the resistance force from microholes, resulting in potential structural failures.36 Besides, consistent fabrication of large-area templates is quite challenging owing to the complication between the applied pressure and the viscosity of the pre-polymer. 37 Inthis paper, we present a self-releasing photolithographic technique for large-area fabrication of structured templates. Beforpatterning templates, the surface modication with specic silanes is used to turns the substrate to be hydrophobic.This low energy surface enables the structured template to selfrelease during the development.This procedure is naturally combined into the conventional photolithography processes, eliminating the need of mechanical separation or any sacricial layers.Negative resist SU-8 and the photo-denable polyimide are selected for microhole template fabrication.With the full advantage of photolithography, this method paves a simple way of high-delity fabrication of templates for their application in micro/nano fabrication, chemistry and biomedical science.

Experimental
Two types of the negative photoresists used in our experiment are SU8-3010 (Microchem, USA) and the photo-denable polyimide precursor HD 4100 (HD Microsystems™, USA).Notably, SU-8 is widely used in nanofabrication, microuidics and so lithography due to its chemical and mechanical stability; the templates with nontoxic and well biocompatible polyimide could benet biological or biomedical applications.
A brief illustration of template fabrication process is shown in Fig. 1a.A substrate of silicon wafer is rst cleaned by heated Nano-Strip (Cyantek, CA, USA) at 85 °C to remove any contaminations including organic and inorganic particles and rinsed by DI water.Aer the dehydration on hotplate at 200 °C for 5 minutes, the wafer surface is silanized using alkyl silanes under vacuum for 1 h to decrease the surface energy to enable selfpeeling of patterned template during resist development.As an example of the typical procedure, SU-8 3010 is spin-coated at 500 rpm for 5 seconds followed by 1000 rpm for 30-35 seconds onto the silanized wafer using a spin-coater (Solitec 5110 Spinner) for the nal template thickness around 15 mm.Aer so baking at 95 °C for 5 minutes, the resist-coated wafer is exposed by the 365 nm UV light for an energy dose of 180 mJ cm −2 through a pre-designed photomask in a contact lithographic system (Karl SyssMA6 Mask Aligner, UV intensity of 12 mW cm −2 ).The post-baking is performed at 65 °C for 2.5 minutes followed by 95 °C for another 2.5 minutes directly aer exposure.The resist layer is then developed in the SU-8 developer solution in a Petri dish.The crosslinked template gradually self-peels from the wafer surface without any other intervention.The synthesized freestanding template is cleaned again with fresh SU-8 developer and isopropyl alcohol, and nally stored in a ventilated cabinet for slow dehydration to prevent it from wrinkling.Fig. 1b demonstrates the freestanding and exible templates of SU8 and the polyimide in wafer scale.

Technical discussion
The same fabrication procedures with different parameters are applicable for SU8 and polyimide respectively.However, two major issues exist in the fabrication: (1) the low adhesion due to silanization could prevent uniformly spin-coating resist layers; (2) overexposure may lead to saturated structures.Thus, the silanization and exposure dose need to be adjusted according to  First of all, the proper silanization of substrate surfaces is essential for large-area photolithographic fabrication of freestanding templates.The substrate needs to be treated to certain degree of hydrophobicity according to the used photoresist, whereas inappropriate treatment may cause non-uniform resist spin-coating or failure in the nal self-releasing of patterned templates.
Trichloro-(1H,1H,2H,2H-peruorooctyl)silane (FOTS) and n-octadecyltrichlorosilane (n-OTS) are compared in our silanization test by spin-coating the selected photoresists on substrates.Fig. 2a and b show the photoresist dewetting on the wafer surfaces silanized by FOTS, indicating the excessively low surface energy.The size comparison of dewetting areas implies that SU-8 (Fig. 2a) is more adhesive to the substrate with the same degree of hydrophobicity than the polyimide (Fig. 2b).In contrast, the wafer surface treated with n-OTS show less hydrophobicity, thereby facilitating the resist spin-coating and archiving the complete surface cover, as shown in Fig. 2c.Notice that temperature should be slowly increased in so baking, otherwise the dramatic temperature rise can crack the resist layer near the edge, as shown in Fig. 2d.Moreover, the baking temperature should been set according to the recommendation to guarantee the thermal stability of the resist. 38o obtain high-delity perforated structures in templates, the UV dose control is another critical measure.On the one hand, the complete crosslink throughout the negative photoresist layer requires a minimum energy dose, under which the catalyzer can reach the bottom region, enabling crosslink of the entire photoresist layer.On the other hand, overexposure may further cure the resist of the undesired area, leading to template structure failure.For instance, the holes that are supposed to perforate the template become closed, as shown in Fig. 3a.Since the polyimide is more susceptible to UV energy variation, the exposure test is carried out to obtain the optimum dose for templates with the hole feature as small as 5 mm (Fig. 3).In contrast, SU8 templates has a larger window of exposure dose as SU8 is relatively less sensitive to overexposure.However, SU8 template tends to be more adhesive to substrate surfaces.Slight overexposure (e.g.200 mJ cm −2 for the thickness of 15 mm) can strengthen its adhesion to substrates, thereby impeding the nal delamination of the structured template.
Others factors such as baking times are also controlled according to the template thickness and photoresists.In traditional photolithography, as the cured photoresist remains on substrates to form micro/nanostructures, the longer baking time is usually helpful to enhance the strength of nal structures.In our case, for the sake of releasing the crosslinked template from substrate, the post-baking is optimized to such an extent that guarantees both crosslinking and self-peeling of the template.Especially, post-baking time needs to be taken good control for SU8, because over-baking can signicantly strength its adhesion to the substrate.

Characterization of templates
For uniform patterning of materials, the high-quality template needs to possess well-dened hole size, shape and distribution.This requirement is accurately realized in our templates with the optimized process.The patterned hole sizes range from several micrometers to several tens of micrometers.Correspondingly, the porosity of templates can reach ∼10 5 holes per cm 2 .Fig. 4 show the SEM images of freestanding templates made of SU8 and the polyimide.For the high-resolution SU8 template, 14 mm diameter of the round holes has only a standard deviation of ∼100 nm based on a random measurement of 10 holes (Fig. 4e).The coefficient of variation is calculated to be   photoresist (Fig. 4b and d).Note, as the polyimide is sensitive to UV energy variation, a bit lower dose is applied to ensure all the microholes open.However, this underexposure also results in the rough edges of the square holes in the polyimide template (Fig. 4c and d).
Further, the template surface is also required to be smooth and exible to ensure a close attachment to the target substrate for material deposition.The surface prole of our templates is quantied using atomic force microscopy (AFM).The roughness of the SU8 bottom surface (Fig. 5a) are within ±30 nm (Fig. 5b).The root mean square of roughness over a 4 × 4 mm 2 are 13.3 nm for the bottom surfaces.Our thin templates show remarkable exibility in the bending and stretching test as well.The insert in Fig. 5c shows the template upon bending, in which it remains intact aer 50 cycles.Usually, such a thin template without a support could be easily distorted or even damaged due to internal tension.In contrast, the microholes in our template maintain their original shape and arrangement even under threefold bending (Fig. 5c and d).By comparison, the polyimide template turns out to be more extendable with higher mechanical strength, whereas the SU8 template is more exible.

Material patterning with templates
We demonstrate the versatility of our templates by material patterning in both liquid and gas phases.The distribution of nanoparticles can be conned by their solution deposition through microhole templates.An SU8 template and a glass slide are rst treated with oxygen plasma immediately before their contact, enabling a close bond between them.100 nm uorescent polystyrene particles in anhydrous ethanol are dropped on the template to generate a pattern enriched with nanoparticles on the glass surface aer ethanol evaporation.As shown in Fig. 6a, nanoparticles are allocated in an array according to the template without leakage.Besides, our templates are also applicable to bulk material patterning through physical vapor deposition.For example, platinum can be sputtered onto the substrate via the SU8 template to produce a micro-disk array (Fig. 6b and c).The uniform array can be generated with precise arrangement and dimensions as those of microholes in large area.

Conclusions
In conclusion, we develop a self-releasing photolithographic technique to synthesize freestanding polymeric templates with precise microholes.Our method eliminates sacricial layers used in previous techniques to enable self-detachment of template from substrates.Instead, the self-releasing relies on the salinization of substrate surfaces to proper hydrophobicity, ensuring both uniform spin-coating of resists and detachment of templates.Moreover, this process is fully compatible with the conventional photolithography, taking all its advantages to engineer specic microstructures, e.g. an extremely low variation.Major steps including UV exposure and baking are also precisely controlled to realize high-quality patterning.As a result, waferscale templates with uniform microhole arrays are obtained with high structural delity, smooth surfaces and excellent exibility.Our templates are capable of patterning nanoparticles and bulk material by solution deposition and physical vapor deposition respectively.These advantages could extend the application of precisely structured templates for surface patterning of various materials in more scientic and engineering areas.

Fig. 1
Fig. 1 (a) Schematic of template fabrication by self-releasing photolithography.(b) Wafer-scale freestanding microhole templates made of SU8 (top) and polyimide (bottom).(c) Structured template self-releasing process: (i) self-detaching during development, (ii) freestanding on wafer surface in the developer, and (iii) template supported by wafer surface after rinsing.
different adhesion and optical sensitivity of two resists, respectively.

Fig. 3
Fig.3UV dose control for high-fidelity fabrication of polyimide templates with microholes: exposure energy decreasing for scanning electron microscopy (SEM) images from (a) to (d).

Fig. 4 Fig. 5
Fig. 4 SEM images of top and bottom of SU8 (a and b) and polyimide (c and d) templates.The diameter of round SU8 hole and the edge of square polyimide holes are 14 mm and 12 mm, respectively.(e) Size measurement of each 10 holes from top and bottom of SU8 and polyimide templates respectively.

Fig. 6
Fig. 6 Patterning polystyrene nanoparticles and metal disks with microhole templates.(a) Fluorescent image of nanoparticle distribution in an array according to the used SU8 template.(b) Microscopy image of large-area platinum micro-disk array and enlarged image (c) with precise arrangement and dimensions.